Both Viking River abd Avalon Waterways have some of the newest boats and best rates you will find. Uniworld is a good company, too, but these others have tons of experience and now cater to the American audience so they know exactly what you want in a river cruise.
I just did an article on Viking River:
Viking River Cruises - Better Than Ever by Paul Motter
Americans flocking to European river cruises in record numbers? What's the secret behind the success? Go>
And I also recently did one on Avalon - you decide and let us know:
Avalon Waterways: Next-Generation River Boats
21st-century European river boats offer the amenities of modern cruise ships; Internet and balconies